Karalee Jarvis

Spinel-type lithium cobalt oxide as a bifunctional electrocatalyst for the oxygen evolution and oxygen reduction reactions

Development of efficient, affordable electrocatalysts for the oxygen evolution reaction and the oxygen reduction reaction is critical for rechargeable metal-air batteries. Here we present lithium cobalt oxide, synthesized at 400 °C (designated as LT-LiCoO2) that adopts a lithiated spinel structure, as an inexpensive, efficient electrocatalyst for the oxygen evolution reaction. The catalytic activity of LT-LiCoO2 is higher than that of both spinel cobalt oxide and layered lithium cobalt oxide synthesized at 800 °C (designated as HT-LiCoO2) for the oxygen evolution reaction. Although LT-LiCoO2 exhibits poor activity for the oxygen reduction reaction, the chemically delithiated LT-Li1-xCoO2 samples exhibit a combination of high oxygen reduction reaction and oxygen evolution reaction activities, making the spinel-type LT-Li0,5CoO2 a potential bifunctional electrocatalyst for rechargeable metal-air batteries. The high activities of these delithiated compositions are attributed to the Co4O4 cubane subunits and a pinning of the Co(3+/4+):3d energy with the top of the O(2-):2p band.

Graphite oxide as a carbocatalyst for the preparation of fullerene-reinforced polyester and polyamide nanocomposites

Graphite oxide (GO) was discovered to catalyze the ring opening polymerization of various cyclic lactones and lactams, such as e-caprolactone, δ-valerolactone, and e-caprolactam, to their corresponding polyesters or polyamides. The resulting polymers were obtained in moderate number average molecular weights (4.8–12.8 kDa) and in good to excellent yields (39–100%) at GO loadings ranging from 2.5–20.0 wt%. Using powder X-ray diffraction (PXRD) and transmission electron microscopy (TEM), it was determined that the carbon catalyst was retained and homogeneously dispersed within the polymer product, resulting in the formation of a carbon-filled composite. TEM also revealed that the carbon transitioned from the lamellar morphology found in GO primarily to nanometre-sized, multiwalled fullerenes; no other discrete carbon morphologies were observed. The inclusion of the carbon material in the polyesters was found to improve the mechanical stiffness of the polymers by up to 400%, as compared to the neat homopolymer.

Radio Frequency Transistors and Circuits Based on CVD MoS2

We report on the gigahertz radio frequency (RF) performance of chemical vapor deposited (CVD) monolayer MoS2 field-effect transistors (FETs). Initial DC characterizations of fabricated MoS2 FETs yielded current densities exceeding 200 μA/μm and maximum transconductance of 38 μS/μm. A contact resistance corrected low-field mobility of 55 cm(2)/(V s) was achieved. Radio frequency FETs were fabricated in the ground-signal-ground (GSG) layout, and standard de-embedding techniques were applied. Operating at the peak transconductance, we obtain short-circuit current-gain intrinsic cutoff frequency, fT, of 6.7 GHz and maximum intrinsic oscillation frequency, fmax, of 5.3 GHz for a device with a gate length of 250 nm. The MoS2 device afforded an extrinsic voltage gain Av of 6 dB at 100 MHz with voltage amplification until 3 GHz. With the as-measured frequency performance of CVD MoS2, we provide the first demonstration of a common-source (CS) amplifier with voltage gain of 14 dB and an active frequency mixer with conversion gain of -15 dB. Our results of gigahertz frequency performance as well as analog circuit operation show that large area CVD MoS2 may be suitable for industrial-scale electronic applications.

Understanding structural defects in lithium-rich layered oxide cathodes

Planar defects in lithium-rich layered oxides were examined by aberration-corrected scanning transmission electron microscopy (STEM) to understand their formation. Planar defects were found to form during the transition of the transition metal layer from a disordered Rm state to a lithium-ordered C2/m state. This disorder-to-order transition resulted in three orientation variants, namely [100], [110], and [10]. The fundamental mechanism behind the observed defects is a shear of ±b/3[010] on the (001) transition metal planes, which is equivalent to the point group operations lost during the disorder-to-order transition. These displacements also produced twins and single unit cells with P3112 symmetry. Lithium-rich layered oxides with and without nickel show the presence of these three orientation variants.

The role of composition in the atomic structure, oxygen loss, and capacity of layered Li–Mn–Ni oxide cathodes

Lithium-rich layered Li[Li1/3−2x/3Mn2/3−x/3Nix]O2 (0 < x ≤ 1/2) oxide cathodes show promise as a potential candidate for Li-ion batteries due to their high capacity. However, the intricacies of the role of composition with increasing excess Li on the degree of oxygen loss during the first charge and the discharge capacity in subsequent cycles are not fully understood. With an aim to develop a better fundamental understanding, we present here an in-depth investigation of the Li[Li1/3−2x/3Mn2/3−x/3Nix]O2 (0 < x ≤ 1/2) series with a range of different excess lithium contents prepared by two different synthesis methods. The oxygen loss from the lattice during the first charge and the discharge capacity in subsequent cycles increase with increasing lithium content. In-depth analysis with a combination of X-ray diffraction, scanning electron microscopy (SEM), aberration-corrected scanning transmission electron microscopy (STEM), diffraction-STEM (D-STEM), and energy dispersive X-ray spectroscopy (EDS) reveals that the samples transition from an Rm structure to a C2/m structure with increasing lithium content and decreasing nickel to manganese ratio, for both the synthesis methods, indicating that the maximum oxygen loss and discharge capacity are achieved with a single C2/m phase. We further show that within a single particle, the cation layers of these materials can order on different {111} planes in the basic NaCl structure.

Effects of Uniaxial and Biaxial Strain on Few-Layered Terrace Structures of MoS2 Grown by Vapor Transport

One of the most fascinating properties of molybdenum disulfide (MoS2) is its ability to be subjected to large amounts of strain without experiencing degradation. The potential of MoS2 mono- and few-layers in electronics, optoelectronics, and flexible devices requires the fundamental understanding of their properties as a function of strain. While previous reports have studied mechanically exfoliated flakes, tensile strain experiments on chemical vapor deposition (CVD)-grown few-layered MoS2 have not been examined hitherto, although CVD is a state of the art synthesis technique with clear potential for scale-up processes. In this report, we used CVD-grown terrace MoS2 layers to study how the number and size of the layers affected the physical properties under uniaxial and biaxial tensile strain. Interestingly, we observed significant shifts in both the Raman in-plane mode (as high as -5.2 cm(-1)) and photoluminescence (PL) energy (as high as -88 meV) for the few-layered MoS2 under ∼1.5% applied uniaxial tensile strain when compared to monolayers and few-layers of MoS2 studied previously. We also observed slippage between the layers which resulted in a hysteresis of the Raman and PL spectra during further applications of strain. Through DFT calculations, we contended that this random layer slippage was due to defects present in CVD-grown materials. This work demonstrates that CVD-grown few-layered MoS2 is a realistic, exciting material for tuning its properties under tensile strain.

Sub-stoichiometric germanium sulfide thin-films as a high-rate lithium storage material

Nanocolumnar, sub-stoichiometric germanium sulfide thin-films with compositions of Ge0.9S0.1 and Ge0.95S0.05, deposited by glancing angle deposition, were investigated as lithium storage materials. The materials are amorphous and homogeneous as deposited, but lithiation induces phase separation leading to the formation of poorly-crystallized lithium sulfide inclusions during the first cycle. The presence of these inclusions raises the lithium diffusion coefficient above that of pure germanium and provides superior capacity retention at high rates. While the lithium sulfide is non-cycling, the low weight percentage of sulfur necessary for enhanced lithiation/de-lithiation does not significantly reduce the specific lithium storage capacity of the films relative to that of germanium. In addition to high capacity and superior lithium transport, the sub-stoichiometric germanium sulfide thin-films show excellent cycling stability at high rates, retaining 88% of their initial capacity after 500 cycles at a rate of 20 C.

Association of Type 2 Diabetes with Submicron Titanium Dioxide Crystals in the Pancreas

Pigment-grade titanium dioxide (TiO2) of 200-300 nm particle diameter is the most widely used submicron-sized particle material. Inhaled and ingested TiO2 particles enter the bloodstream, are phagocytized by macrophages and neutrophils, are inflammatory, and activate the NLRP3 inflammasome. In this pilot study of 11 pancreatic specimens, 8 of the type 2 diabetic pancreas and 3 of the nondiabetic pancreas, we show that particles comprising 110 ± 70 nm average diameter TiO2 monocrystals abound in the type 2 diabetic pancreas, but not in the nondiabetic pancreas. In the type 2 diabetic pancreas, the count of the crystals is as high as 108-109 per gram.

Effects of Grain Boundaries and Defects on Anisotropic Magnon Transport in Textured Sr14Cu24O41

The strong spin-spin exchange interaction in some low-dimensional magnetic materials can give rise to a high group velocity and thermal conductivity contribution from magnons. One example is the incommensurate layered compounds (Sr,Ca,La)14Cu24O41. The effects of grain boundaries and defects on quasi-one-dimensional magnon transport in these compounds are not well understood. Here we report the microstructures and anisotropic thermal transport properties of textured Sr14Cu24O41, which are prepared by solid-state reaction followed by spark plasma sintering. Transmission electron microscopy clearly reveals nano-layered grains and the presence of dislocations and planar defects. The thermal conductivity contribution and mean free paths of magnons in the textured samples are evaluated with the use of a kinetic model for one-dimensional magnon transport, and found to be suppressed significantly as compared to single crystals at low temperatures. The experimental results can be explained by a one-dimensional magnon-defect scattering model, provided that the magnon-grain boundary scattering mean free path in the anisotropic magnetic structure is smaller than the average length of these nano-layers along the c axis. The finding suggests low transmission coefficients for magnons across grain boundaries.

Interfacial reactions at Fe/topological insulator spin contacts

The authors study the composition and abruptness of the interfacial layers that form during deposition and patterning of a ferromagnet, Fe on a topological insulator (TI), Bi2Se3, Bi2Te3, and SiOx/Bi2Te3. Such structures are potentially useful for spintronics. Cross-sectional transmission electron microscopy, including interfacial elemental mapping, confirms that Fe reacts with Bi2Se3 near room temperature, forming an abrupt 5 nm thick FeSe0.92 single crystalline binary phase, predominantly (001) oriented, with lattice fringe spacing of 0.55 nm. In contrast, Fe/Bi2Te3 forms a polycrystalline Fe/TI interfacial alloy that can be prevented by the addition of an evaporated SiOx separating Fe from the TI.